1. Field of the Invention.
This invention is related to compound semiconductor growth and device fabrication. More particularly the invention relates to the growth and fabrication of indium gallium nitride (InGaN) containing electronic and optoelectronic devices by metalorganic chemical vapor deposition (MOCVD).
2. Description of the Related Art.
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [Ref. x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlINGaN) has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially by growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), or hydride vapor phase epitaxy (HVPE).
GaN and its alloys are most stable in the hexagonal würtzite crystal structure, in which the structure is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. FIG. 1 is a schematic of a generic hexagonal würtzite crystal structure 100 and planes of interest 102, 104, 106, 108 with these axes 110, 112, 114, 116 identified therein, wherein the fill patterns are intended to illustrate the planes of interest 102, 104 and 106, but do not represent the materials of the structure 100. Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the wuirtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis. Furthermore, as the würtzite crystal structure is non-centrosymmetric, würtzite nitrides can and do additionally exhibit piezoelectric polarization, also along the crystal's c-axis. Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electron and holes that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
(Al,Ga,In)N quantum-well structures employing nonpolar growth directions, e.g., the <11 20> a-direction or <1 100> m-direction, provide an effective means of eliminating polarization-induced electric field effects in würtzite nitride structures since the polar axis lies within the growth plane of the film, and thus parallel to heterointerfaces of quantum wells. In the last few years, growth of nonpolar (Al,Ga,In)N has attracted great interest for its potential use in the fabrication of nonpolar electronic and optoelectronic devices. Recently, nonpolar m-plane AlGaN/GaN quantum wells grown on lithium aluminate substrates via plasma-assisted MBE and nonpolar a-plane AlGaN/GaN multi-quantum wells (MQWs) grown by both MBE and MOCVD on r-plane sapphire substrates showed the absence of polarization fields along the growth direction. Thus, nonpolar III-nitride light emitting diodes (LEDs) and laser diodes (LDs) have the potential to perform significantly better compared to their polar counterpart.
Unfortunately, nonpolar InGaN growth has proven challenging. Indeed, the literature contains only two reports of the successful growth of nonpolar InGaN: Sun, et al. [Ref. 1], grew m-plane InGaN/GaN quantum well structures containing up to 10% In by MBE, and Chitnis, et al. [Ref. 2], grew a-plane InGaN/GaN quantum well structures by MOCVD. Sun, et al's, paper [Ref. 1] focused primarily on structural and photoluminescence characteristics of their material, and does not suggest that their InGaN film quality is sufficient to fabricate working devices. Chitnis, et al's paper [Ref. 1] described a nonpolar GaN/InGaN light emitting diode structure. However, the limited data given in the paper suggested their nonpolar InGaN material quality was extremely poor. Indeed, their device displayed large shifts in emission intensity with varying injection current, poor diode current-voltage characteristics, and extreme detrimental heating effects that necessitated pulsing the current injection in order to test the device. These poor characteristics most likely can be explained by deficient material quality.
The lack of successful nonpolar InGaN growth can be attributed to several factors. First, the large lattice mismatches between InGaN and available substrates severely complicate InGaN heteroepitaxy. Second, InGaN must generally be grown at comparatively lower temperatures than GaN due to the propensity for In to desorb from the growth surface at higher temperatures. Unfortunately, nonpolar nitrides are typically grown above 900° C. and more often above 1050° C., temperatures at which In readily desorbs from the surface. Third, high-quality nonpolar nitrides are typically grown at decreased pressures (<100 Torr) in order to stabilize the a- and m-planes relative to inclined facets. However, it has been previously widely reported that c-plane InGaN should be grown at atmospheric pressure in order to enhance In incorporation and decrease carbon incorporation.
The present invention overcomes these challenges and for the first time yields high quality InGaN films and InGaN-containing devices by MOCVD.